Which boiler is best?

That’s easy – the Potterton Profile – rugged, simple, well-designed, reliable and maintainable. If you’ve got one, don’t rush to replace it. However, it belongs to a bygone era.


I’ve written this for people wanting help with choosing a boiler, and I’m afraid I haven’t such a straightforward answer for today, but here are some thoughts that might help you make a choice. This isn’t a politician’s answer – I really am trying to answer the question! The most important thing is to think carefully about your needs and make your own enquiries and decision, rather than fall prey to sales technique.


Type of boiler:


By this I don’t mean manufacturer, but what the boiler does. Below are the main classes and some pros and cons to ponder, but first, let’s get condensing boilers out of the way. With few exceptions for special cases in multiple dwellings, any boiler that you buy today will be of the condensing type, by law. This recovers heat energy from the water vapour in the flue gases that formerly used to be lost, making it more efficient. The condensed liquid water, the “condensate” has to be got rid of into a drain or soak-away, via a condensate pipe. So your new boiler will be a condensing one, and this doesn’t form part of your decision.


[One might pause to wonder why, if they’re such a wonderful idea, condensing boilers had to be made compulsory rather than being adopted naturally on their merits.]


Combination or “combi”: the distinguishing feature of a combi boiler is that the hot water that flows to the taps or shower comes from the cold water main and is instantaneously heated by the boiler. If it’s providing space heating (radiators or underfloor heating), as soon as there’s a hot water demand, it switches over to satisfying the latter.


  • Ideal for small properties with few occupants – no hot water cylinder, cold water cistern or feed & expansion tank (the latter two traditionally located in lofts), and no separate motorised valves.
  • Relatively straightforward to install.
  • Simple control system (no hot water controls).
  • No running out of hot water.


  • More complex so potentially less reliable – the plate-to-plate heat exchanger, though technological marvel, is prone to blocking and scaling up (the latter in hard water areas).
  • If they break down, there’s no hot water backup (no immersion heater). This can really make people miserable.
  • Can’t support a good flow rate from several taps simultaneously.
  • Boiler power is often dictated by hot water flow rate requirement so can end up being much more powerful than needed for space heating.



Regular or heat-only boiler: this the simplest and is closest to a traditional boiler. Water, propelled by an external pump, enters the boiler via the “return” pipe, passes through the heat exchanger which is heated by the burner, and the heated water leaves through the “flow” pipe to heat radiators, a hot water storage cylinder, or both, depending how the control system is configured. They may be employed in an “open-vented” system with a feed & expansion (F&E) tank (generally in the loft) or a sealed system with a separate expansion vessel.


  • Relatively simple so a better chance of being reliable.
  • Can be sized largely for space heating so typically less powerful and smaller.
  • If an immersion heater is fitted to the hot water cylinder, there’s a backup if the boiler’s out of action.
  • Hot water temperature and volume can be controlled separately.
  • Hot water flow rate is independent of boiler power.


  • Space needed for storage cylinder, F&E tank or expansion vessel, and associated pipework.
  • A relatively complex control system and valve(s) are required, so if starting afresh, installation cost would be greater than for a combi.



System boiler: this is like the regular boiler, and works with a stored hot water cylinder, but has more parts of the system within its case. System boilers are designed for sealed systems (with no F&E tank) and contain the expansion vessel and pump. Pros and cons are very similar to those for the regular boiler, but installation cost would be lower for a complete new system (but higher than for a combi), and the boiler itself a little larger to accommodate the expansion vessel and pump.



So, which type?


As I noted above, it’s important that the decision is made by you. At the extremes, the choice is fairly easy. If you live on your own and space is at a premium, the combi is the closest match to your needs. At the other extreme, you have plenty of space, several people in the property, more than one bathroom and a healthy budget, and would like hot water readily available with a good flow rate. In this case, a regular or system boiler heating an unvented hot water storage cylinder with immersion heater backup, with a power appropriate for your property, will deliver what you want. If, like many, you’re between these extremes, you’ll have to do some thinking.



Which boiler manufacturer?


This one is complex: all boilers I’ve come across have their strengths and weaknesses, and complex specifications. To simplify things a little, there are parameters (“bullet points”) that manufacturers quote that look like selling features, but which you can forget about in choosing a brand, because, important as these things are, they’re practically the same for different makes:


  • Hot water flow rate for a combi boiler: this depends only on the boiler power. They’re all so good that flow rate is more or less set by the laws of physics.
  • Efficiency – important of course, but they’re all about 90% (SEDBUK rating – Seasonal Efficiency of Domestic Boilers in the UK) and any differences are as likely to be due to measurement uncertainty as real.


You’re also unlikely to see in manufacturers’ literature:


  • “A real challenge to install” or,
  • “An utter pig to service.”


I don’t install boilers, only service and repair them, but I’ll try to look at things from an installer’s point of view. For him, the price paid for the boiler can be high on the importance scale. If he can shave something off this, it’s clear profit and helps him earn a living from hard work in an environment where the streets aren’t paved with gold. Some installers work with only one or a limited range of manufacturers. They may have attended the manufacturer’s training courses, have become knowledgeable and familiar with particular boilers, enabling them to install with competency, efficiency and confidence, backed by good manufacturer support. Their discount on the boiler price is earned by the number of installations and so the volume of product – boiler manufacture is now highly competitive with small margins. In my view, these installers deserve respect.


Perhaps less worthy are cases of boilers selected solely or primarily on the basis of special offers or deals. These may help explain a small minority of cases where the boiler itself is fine, but the manufacturer has no presence in this country, and the documentation is poor, with perhaps no proper instructions as to how to service the boiler or replace parts.


Here are some questions I’d ask when researching new boilers:


  • Are there particular constraints? These might include such things as:
  • Limited room: for instance, this might mean that a boiler with a rear flue option would fit best. (Be careful here though – some boilers get packed so tightly in small cupboards that service and repair are difficult or impossible.)
  • The need for a particularly quiet boiler, for a bedroom for example. Some manufacturers include data in the technical specifications of the Installation and Service manual. Look for a figure in dBA (A-weighted decibels – the lower the better.)
  • Does the boiler manufacturer have a significant and established presence in this country?
  • Are there customer reviews which appear to be genuine? (Genuine reviews can be expected to have some adverse comments too.) Do certain failures keep getting mentioned, indicating a reliability issue? (Reliability is important enough to deserve its own note, see below.) Internet forums can be useful, but bear in mind that, as on the telly, it’s bad news that tends to get reported.
  • Is the boiler literature readily down-loadable from the manufacturer’s website?
  • Are spares reasonably priced? If you’re prepared to put in some effort, you can do some research on this. Towards the end of the Installation & Service manual, there should be a list of replaceable parts with part numbers. Try looking up online the cost of a new pump if there is one, the control PCB (printed circuit board), fan and gas valve.


Boiler reliability:


Boiler failures outside of warranty can be expensive, and cost much more than the difference in price between two boilers. That noted, it isn’t easy getting useful information.


A major source of boiler failures is poor heating water quality. The impression has been given that improved efficiency is purely down to the condensing aspect of new boilers, but this isn’t the case. Modern heat exchangers transfer more heat from the burning gas to the circulating central heating water than older ones. The price for this is narrower internal water-ways which are therefore more prone to becoming blocked. This is particularly the case with the combi boiler’s plate-to-plate heat exchanger which provides the instantaneous hot water. The moral of this is not to economise or cut corners when it comes to having the heating water cleansed prior to installation of a new boiler. In addition, some manufacturers, if called upon to repair a boiler under warranty, take a sample of the heating water. If it’s not up to standard, the warranty (which could be five years or more) will be void, which could be costly.


One approach to seeking out a manufacturer that can be relied on, is to see what the big boys fit and support, the biggest boy being British Gas. This is on the assumption that a boiler that was too unreliable would too costly both in terms of time and effort consumed and damaged reputation. That noted, I came across a recently installed boiler from one of the big names currently (April 2013) on BG’s website that ignited fairly explosively (not dangerous, but it made something of a noise).


Service & repair:


As this is what I do, this section is written from my selfish perspective! The things I most value in a boiler manufacturer are:


  • Technical support that’s competent, diplomatic and helpful, without lengthy waits on the phone. I know one boiler manufacturer with superb product that’s beautifully designed and easy to work on – a work of art in fact – but poor technical support lets them down – I couldn’t recommend them, at least not without a massive bribe.
  • Boiler design in which thought has been given to maintenance.
  • Quality of documentation (has the person writing the manual actually done what’s being written about?)


I can report that in terms of technical support that meets my criteria, Worcester Bosch stand out clearly from the crowd (that’s not to denigrate excellent people at other companies, but waiting up to an hour to get through to them on the phone can dent the spirits a little, and time is money).


In summary:


  • There’s no simple answer.
  • Think about what you want.
  • Be prepared to do some research.
  • Don’t set too much store by historical information. Over time, any one boiler manufacturer brings out models that are good, and less good. Focus only on what they currently sell.
  • Buy a boiler from a manufacturer with a good pedigree in this country.
  • Before any new boiler is installed, the heating water must be properly cleansed, otherwise any boiler is liable to be unreliable. Cut corners here and there’ll be tears later.





Boiler types:





Condensing boilers: